The following answers to your questions are from my hands-on experiences.
1. Nothing, the Navigation system nor the G-Book works in Pakistan. Those are the critical features, which makes the built-in factory TV/NAV valuable. The benefits of this unit here will be using it's touch-screen climate control system, back-up camera, auto-parking system (if your car is equip with it) and last but not least for attaching a DVD changer in order for it to play DVD's.
NOTE: Vehicles equip with OEM TV/NAV's tend to have a higher after-sales (re-sale) value.
2. The new Android and Pioneer/Sony/Alpine multi-media DVD players have a lot of features these days. For example: Built-in HDD's, Aux, USB, Blu-ray playback, SD card option, auto-save and so on. It is actually better to have an after-market head-unit unless you want to keep the ''originality''.
3. The vehicles without the OEM TV/NAV have physical buttons with a narrow LCD, right below the media player panel. The OEM TV/NAV is a factory option. It came optional in the F, G and even Type S package.
4. You have to deal with the Japanese language format, you will get use to it. As the Mark X is a Japan-only model there are no Japanese to English language conversions available.
5. If properly maintained a 250G Mark X will give you anywhere between 6.5-8.5 Km/L, depending upon your driving style and environment.
6. You can most certainly. Here in Karachi, a decent 2004/2005 Mark X 250G can be bought in between 18-23 lacs. Depending upon the options, mileage, color and overall condition (mechanical and physical).
I hope all your questions have been answered.